Effect of Ultrasonication Duration on Colloidal Structure and Viscosity of Alumina–Water Nanofluid

Nanofluids are promising fluids for heat-transfer applications. Low stability and high viscosity are two important drawbacks for practical applications of nanofluids. The aggregation and sedimentation of nanoparticles are related to the colloidal structure of nanofluids, which directly affects the stability and viscosity. An ultrasonic homogenizer can break the aggregation of particles. The aim of this work was to study the effect of the duration of ultrasonic treatment on colloidal structure, including the stability and temperature-dependent viscosity of a nanofluid. Specifically, a 0.5 vol % Al2O3–water nanofluid was prepared using an ultrasonic homogenizer for various durations from 0 to 180 min. The microstructure, colloid and particle sizes, precipitation, and zeta (ζ) potential were analyzed to investigate the aggregation and sedimentation of the nanofluid. The viscosities of nanofluids subjected to ultrasonic treatment for different durations were also measured at different temperatures from 15 to 45 °C. Better particle dispersion, lower particle sizes, smaller colloid sizes, less precipitation, and higher ζ potentials were observed with increasing sonication time. The viscosity of Al2O3–water nanofluid was found to increase with the sonication time up to 60 min and then subsequently decreased. In addition, the viscosity decreased with increasing temperature. The research concluded that more stabler and lower-viscosity nanofluids can be obtained by applying ultrasonic treatment for durations of 90 min or longer

Energy, Exergy and Friction Factor Analysis of Nanofluid as a Coolant for Electronics

Power dissipation, chip power consumption, and heat flux in electronic devices have been steadily increasing over the past decade, creating a need for improved methods of cooling them. Nanofluids can be used as coolant for these electronics to improve their thermal performance. This paper presents an analysis of the energy, exergy, and frictional efficiencies of different nanofluids that are used to cool electronics. This was done by creating an analytical model in which different nanofluids flowed (at 0.5 m/s) through a rectangular-shaped microchannel heat sink (with a constant heat flux). These different nanofluids consisted of water as a base fluid, with 0.4 to 2.0 vol % of copper oxide (CuO), aluminum oxide (Al2O3), and titanium dioxide (TiO2) nanoparticles. The results generally showed that thermal resistance decreases as the volume fraction of nanoparticles is increased. The CuO-water nanofluid was found to be the best coolant in terms of both minimizing thermal resistance and maximizing the pressure reduction. The energy efficiency of the heat sink increases as the volume fraction of nanoparticles increases. A maximum energy efficiency of 98.9% was obtained using the CuO-water nanofluid (at 2.0 vol %). The Al2O3-water and TiO2-water nanofluids (also at 2.0 vol %) produced a maximum energy efficiency of 77.5% and 68.4%, respectively. The lowest exergy losses were: 19.2, 20.9, and 25.1 W for TiO2-water, Al2O3-water, and CuO-water nanofluids (all at 0.4 vol %), respectively. The dimensionless friction factor was reduced as the nanoparticle volume concentration increased. Also, the pumping power increased (to a high of 0.0173 W) as the mass flow rate increased